Often used in industrial measurements technology, especially in connection with control and monitoring of automated, technical processes, for ascertaining characteristic process parameters, for example a mass flow, a density, a viscosity, etc., of media flowing in a pipeline, for example liquids and/or gases, are in-line measuring devices, especially in-line measuring devices in the form of mass flow meters, which, by means of a measuring transducer of vibration-type and an operating and evaluating electronics connected thereto, induce forces in the flowing medium, for example Coriolis forces, and derived from these, produce a measurement signal measurement signal representing at least one parameter. Such in-line measuring devices having a measuring transducer of vibration-type are long known and well established in industrial usage. Examples of such measuring transducers, especially also their application in Coriolis mass flow meters, are described e.g. in EP-A 317 340, U.S. Pat. No. 4,738,144, U.S. Pat. No. 4,777,833, U.S. Pat. No. 4,823,614, U.S. Pat. No. 5,291,792, U.S. Pat. No. 5,398,554, U.S. Pat. No. 5,476,013, U.S. Pat. No. 5,602,345, U.S. Pat. No. 5,691,485, U.S. Pat. No. 5,796,010 U.S. Pat. No. 5,796,012, U.S. Pat. No. 5,945,609, U.S. Pat. No. 5,979,246, U.S. Pat. No. 6,330,832, U.S. Pat. No. 6,397,685, U.S. Pat. No. 6,691,583, U.S. Pat. No. 6,840,109, U.S. Pat. No. 6,883,387, U.S. Pat. No. 7,077,014, U.S. Pat. No. 7,017,424, U.S. Pat. No. 7,299,699, US-A 2007/0186685, US-A 2007/0151371, US-A 2007/0151370, US-A 2007/0119265, US-A 2007/0119264, WO-A 99 40 394, WO-A 01 02 816 or WO-A 00 14 485. Each of the measuring transducers shown therein includes at least one, essentially straight, or at least one curved, measuring tube, which vibrates during operation and serves for conveying the medium. The measuring tube communicates with the pipeline via an inlet tube piece on the inlet side of the measuring tube and an outlet tube piece on the outlet side of the measuring tube.
Additionally, each of the disclosed measuring transducers includes at least one tubular, box-shaped or plate-shaped counteroscillator, which is embodied as one piece or multi-piece, coupled to the measuring tube on the inlet side to form a first coupling zone and on the outlet side to form a second coupling zone, and likewise caused to vibrate, at least in part, during operation. In the case of the measuring transducers shown in U.S. Pat. No. 5,291,792, U.S. Pat. No. 5,796,010, U.S. Pat. No. 5,945,609, U.S. Pat. No. 7,077,014, US-A 2007/0119262, WO-A 01 02 816 or also WO-A 99 40 394 having a single, essentially straight, measuring tube, such tube and the counteroscillator are, as quite usual in the case of conventional, industrial-grade measuring transducers, directed essentially coaxially with one another. Moreover, in the case of commonly marketed measuring transducers of the aforementioned kind, also the counteroscillator is, most often, essentially tubular and essentially straight and, additionally, arranged in the measuring additionally, arranged in the measuring transducer in such a manner that the measuring tube is at least partially surrounded by the counteroscillator, and such that measuring tube and counteroscillator are essentially coaxially directed. Materials used for such counteroscillators include, among others, comparatively cost-favorable steel types, such as, perhaps, structural steel or free-machining steel.
Measuring transducers of the kind discussed here include, additionally, an exciter mechanism, which, driven by an appropriately conditioned, electrical driver signal, excites the measuring tube during operation by means of at least one electromechanical, especially electrodynamic, oscillation exciter to execute bending oscillations, usually, as much as possible, predominantly or exclusively, in a single, imaginary, tube oscillation plane, hereinafter referred to as the primary plane of oscillation and imaginarily passing through the two coupling zones. Additionally, such measuring transducers include a sensor arrangement having oscillation sensors, especially electrodynamic oscillation sensors, for the at least pointwise registering of inlet-side and outlet-side oscillations of the measuring tube and for producing electrical, sensor signals influenced by the mass flow.
The exciter mechanism includes at least one electrodynamic, oscillation exciter and/or an oscillation exciter differentially acting on measuring tube and counteroscillator, while the sensor arrangement includes an inlet-side, most often likewise electrodynamic, oscillation sensor, as well as an outlet-side, oscillation sensor of essentially equal construction. In the case of usually marketed measuring transducers having a single measuring tube and a counteroscillator coupled thereto, the oscillation exciter is formed usually by means of a coil, through which an electrical current flows, at least at times, and through which a magnetic field passes, at least at times, as well as by means of a rather elongated, especially rod-shaped, permanent magnet serving as armature, interacting with, especially plunging in, the at least one coil, and being appropriately affixed to the measuring tube. Permanent magnet and coil are, in such magnet and coil are, in such case, usually so oriented that they extend essentially coaxially with respect to one another.
Additionally, in the case of conventional measuring transducers, the exciter mechanism is usually constructed and placed in the measuring transducer in such a manner that it acts essentially centrally on the measuring tube. Most often, the at least one oscillation exciter and, as a result, the exciter mechanism, is, in such case, additionally, as shown, for example, also in the measuring transducers disclosed in U.S. Pat. No. 5,796,010, U.S. Pat. No. 6,840,109, U.S. Pat. No. 7,077,014 or U.S. Pat. No. 7,017,424, affixed externally to the measuring tube, at least pointwise along an imaginary, central, peripheral line thereof. Alternatively to an exciter mechanism formed by means of oscillation exciters acting rather centrally on the measuring tube, for example, as proposed in, among others, U.S. Pat. No. 4,823,614, an exciter mechanism can be applied, which is formed by means of two oscillation exciters affixed to the measuring tube, not in the center of thereof, but instead more toward the inlet and outlet ends thereof.
In the case of most measuring transducers of the described kind, the oscillation sensors of the sensor arrangement are, as already indicated, constructed according to the same principle, at least to the extent that they are embodied with essentially equal construction as the at least one oscillation exciter. Accordingly, also the oscillation sensors of such a sensor arrangement are formed, most often, in each case, by means of at least one coil usually affixed to the counteroscillator. At least at times, a varying magnetic field also passes through this coil and, associated therewith, the coil bears, at least at times, an induced, measurement voltage. Additionally, these oscillation sensors each also include a permanently magnetic armature, which is affixed to the measuring tube. interacts with the at least one coil, and supplies the magnetic field. Each of the aforementioned coils is, additionally, connected by at least one pair of electrical connection lines with the mentioned operating- and evaluating-operating- and evaluating-electronics of the in-line measuring device. The connection lines are, most often, guided on the shortest possible path from the coils, along the counteroscillator, to the transducer housing.
For homogenizing the magnetic field passing through coils and permanent magnets, as well as for preventing disturbing stray fields, oscillation sensors of the aforementioned kind as well as also most oscillation exciters have the permanent magnet placed within a magnet cup made at least partially of magnetically conductive material. The permanent magnet is mounted there to a cup base usually directly secured to the measuring tube. Extending from the cup base in the direction of the relative oscillations of measuring tube and counteroscillator is a tubular, essentially circularly cylindrical, cup wall of the magnet cup. Usually, the permanent magnet is arranged essentially in a center of the cup base and, most often, so affixed thereto that permanent magnet and cup wall are oriented to extend essentially coaxially with one another.
Besides the oscillation sensors provided for registering vibrations of the measuring tube, the measuring transducer can, as also proposed, among other things, in EP 831 306, U.S. Pat. No. 5,736,653, U.S. Pat. No. 5,381,697 or WO-A 01/02 816, include still other sensors arranged on the inner part formed, in any case, by means of measuring tube, counteroscillator, as well as the exciter mechanism and sensor arrangement, provided, in each case, thereon or also in their proximity, and serving especially for registering rather secondary measured variables, such as e.g. temperature, acceleration, strain, stress, etc.
Finally, each of the measuring transducers shown in U.S. Pat. No. 5,291,792, U.S. Pat. No. 5,945,609, U.S. Pat. No. 7,077,014, US-A 2007/0119264, WO-A 01 02 816 or also WO-A 99 40 394 includes an extra, transducer housing surrounding the measuring tube, with counteroscillator coupled thereto, as well as the provided exciter mechanism and and sensor arrangement, especially such a transducer housing affixed directly to the inlet tube piece and to the outlet tube piece, while, for example, in the case of the measuring transducer shown in U.S. Pat. No. 4,823,614, the transducer housing is formed quasi by the counteroscillator, or, in other words, transducer housing and counteroscillator are one and the same component.
An advantage of measuring tranducers with straight measuring tube, in comparison to those with curved, or angled, measuring tube, is e.g. that the straight measuring tube empties, to a high degree of certainty, in almost any installed orientation, especially also following an in-line-conducted cleaning. Additionally, such measuring tubes are significantly easier and accordingly more cost favorable to manufacture, as compared e.g. to a curved measuring tube, while, in operation, they, most often, result in a lesser pressure drop.
A straight measuring tube, as is known, brings-about Coriolis forces, when it is excited to execute bending oscillations in the primary oscillation plane according to a first form of eigenoscillation—the so-called drive-mode, or also, wanted-mode. In the case of conventional measuring transducers of the aforementioned type, for example also those disclosed in U.S. Pat. No. 5,291,792, U.S. Pat. No. 6,840,109, U.S. Pat. No. 7,077,014 or U.S. Pat. No. 7,017,424, when the measuring tube is caused to oscillate in the wanted mode mainly in the imaginary, primary plane of oscillation, these Coriolis forces lead, in turn, to the fact that, superimposed on the same bending oscillations in the wanted mode are coplanar (thus, executed likewise in the primary plane of oscillation) bending oscillations according to a second form of eigenoscillation of, most often, higher order, in any case, however, of other symmetry characteristics (the so-called Coriolis-, or also, measuring-mode). As a result of the bending oscillations in the Coriolis mode, the oscillations registered inlet-side and outlet-side by means of the sensor arrangement exhibit a measurable phase difference dependent also on mass flow.
Usually, the measuring tubes of such measuring transducers, especially those utilized in Coriolis mass flow meters, are excited in the wanted mode to an instantaneous resonance frequency of a first form of eigenoscillation, especially at oscillation amplitude controlled to be constant. Since this resonance frequency depends, especially, also on the instantaneous density of the medium, at least also the density of flowing media can be directly measured by means of usually marketed Coriolis, mass flow meters.
Besides the above-mentioned, more or less marked density-dependence, a special problem of measuring transducers as above described with straight measuring tube lies, however, therein (and this is also discussed, for example, in U.S. Pat. No. 5,291,792, U.S. Pat. No. 7,077,014 or the not-prepublished, German patent application 102007050686.6 of the assignee), that they exhibit not only the above-discussed, natural modes of oscillation, in which the measuring tube executes bending oscillations in the mentioned, primary plane of oscillation, but also natural modes of oscillation, in which the measuring tube can execute bending oscillations in another imaginary, secondary plane of oscillation essentially orthogonal to the primary plane of oscillation and equally imaginarily cutting through the two coupling zones, and that, without the accessing of special measures, these modes of oscillation in the secondary plane of oscillation can naturally exhibit about the same resonance frequency as possessed by the respectively corresponding mode of oscillation in the primary plane of oscillation. In other words, in the case of measuring transducers of the type being discussed, with straight measuring tube, possible inaccuracies of measurement, especially based on changes of the zero-point unpredictable during operation, can result from the fact that, in addition to the desirably excited, wanted mode in the primary plane of oscillation, undesired and, thus, disturbing oscillations occur in the secondary plane of oscillation and lie close to the frequencies of oscillation of the wanted mode. Equally as for the wanted mode in the primary plane of oscillation, there would then also be induced, for the equal-frequency modes of oscillation in the secondary plane of oscillation excited in undesired manner, additional in undesired manner, additional modes of oscillation coplanar therewith, related to corresponding Coriolis forces. A cause of such disturbances can be, for example, vibrations in the connected pipeline or, also, most-often broadband noise stemming from the flowing medium. As a result of, in practice, almost unavoidable, transverse sensitivities of the oscillation sensors to oscillations in the secondary plane of oscillation, this leads to the fact that the sensor signals delivered under such circumstances reflect, in part, both oscillations of the measuring tube in the primary plane of oscillation as well as also corresponding oscillations of the measuring tube in the secondary plane of oscillation, to a degree significant for accuracy of measurement. A matching of the corresponding signal parts to the primary and secondary planes of oscillation is, practically, not possible, because the oscillations have essentially equal frequencies. Moreover, in the case of sufficiently strong, mechanical coupling of the oscillatory modes of the two planes of oscillation, also a transfer of oscillatory energy is possible, spontaneously or periodically, from the primary into the secondary plane of oscillation, or also the other way around, from the secondary into the primary plane of oscillation.
As a result of this, the sensor signals can exhibit, for example, a characteristic beat quite damaging both for their signal processing as well as also for oscillation control based on the sensor signals. Furthermore, oscillatory motions in the secondary plane of oscillation, be they excited directly by external disturbances or indirectly via the aforementioned energy transfer from the primary into the secondary plane of oscillation, can lead to the fact that the sensor signals can exhibit an, at times, overly high signal level, with the result that the input amplifier receiving and processing the sensor signals must be, correspondingly, over dimensioned and, consequently, comparatively expensive.
For suppressing such, on the whole, very damaging oscillations executed in the secondary plane of oscillation, it is usual to increase a stiffness of the measuring tube effective for these oscillations relative to a stiffness of the measuring tube effective for oscillations in the primary plane of oscillation, while keeping effective masses essentially equal, and, so, to effectively separate from one another, resonance frequencies of corresponding modes of oscillation of primary and secondary planes of oscillation. Typically, in such case, frequency separations of more than 30 Hz are sought. In U.S. Pat. No. 5,602,345, for this, it is proposed, for example, to apply spring elements in the form of flat struts placed additionally on the particular measuring tube on the inlet and outlet sides in the immediate vicinity of the respective coupling zones. A further possibility for separating oscillation modes in the primary plane of oscillation from corresponding modes of oscillation in the secondary plane of oscillation is additionally disclosed in U.S. Pat. No. 5,291,792. In the measuring transducer proposed there, the stiffness of the measuring tube effective for oscillations in the secondary plane of oscillation is increased by biasing the measuring tube at its center with a correspondingly acting, spring element in the form of an, in such case, U-shaped, stiffening spring arranged extending in the measuring transducer essentially in radial direction to measuring tube and counteroscillator. This spring element does not influence the stiffness of the measuring tube for the Coriolis mode in the primary plane of oscillation to any extent worth mentioning. In this way, it is possible to achieve that the oscillation frequency of oscillations in the wanted mode rises sufficiently strongly above the frequency of undesired, thus disturbing, oscillations, so that the influence of such disturbing oscillations is largely suppressed. Alternatively to this, in the mentioned German patent application 1020070500686.6, it has been proposed to use “decentralized” spring elements placed on the inlet and outlet sides in the vicinity of the coupling zones for frequency separation.
As discussed in the non-prepublished, German patent applications 102006062220.0, 102006062219.7, or 102006062185.9 of the assignee, it has additionally been possible—especially also in the case of an inner part perfectly balanced as regards density, at least under laboratory conditions, and caused to oscillate solely in the primary oscillation plane—to identify the connection lines as a further source for such disturbances of the oscillation measurement signals, especially disturbances affecting also the zero point. Taking this into account, it is proposed in these patent applications to counteract such disturbances by a specially suited leading of the lines along the inner part, out to the transducer housing.
Although the aforementioned measures, taken singly or in combination, have led to quite significant improvements of the measuring accuracy of measuring transducers of the type being discussed, especially also as regards their zero point stability, further investigations, especially investigations carried out also under laboratory conditions and largely free of disturbing vibrations have still led to the detection of fluctuations in the zero point, which, although small, are nevertheless not insignificant for the extremely high accuracy of measurement sought-after for such measuring transducers, and it has not been possible to explain these fluctuations on the basis of any of the above-mentioned phenomena. Especially, it has been found that, despite extensive elimination or prevention of the above-mentioned disturbances, still there is a certain dependence of the zero point on the installation situation, which, in turn, shows a certain dependence on location.
Other disturbance sources potentially degrading the measuring accuracy, especially the stability of the zero point, of measuring transducers of the type discussed, sources such as electromagnetic, alternating fields, or, as discussed, among others, in U.S. Pat. No. 7,299,699, oscillatory rubbing, material fatigue, or loosening of component connections, could, in such case, likewise, be eliminated or would not be able to explain at least the degree of the observed shiftings of the zero point.
Laboratory experiments with a Helmholtz coil, involving exposing a measuring transducer of the type being discussed, installed in various positions, to the switched magnetic field (known to be largely homogeneous) of the Helmholtz coil have finally, surprisingly, identified constant magnetic fields as a possible disturbance source for the long inexplicable, high, observed shiftings of the zero point. Taking this further, it was, thus, finally possible to discover also the special influence of the earth's magnetic field, which is location-dependent, in the above sense, to a considerable degree, as the cause for a locational dependence of the zero point, or, much more, a locational dependence of its changes. Considering the rather high field strengths of about 800 mT, which bring-about the regular measuring voltages in the oscillation sensors, and in view of the fact that the earth's magnetic field is weaker by some orders of magnitude, the sensitivity of the oscillation sensors to local changes of the earth's magnetic field density is quite surprising.
Now, a possibility for removing the aforementioned problem would be available, for example, in the direction of so constructing the transducer housing that its effective magnetic resistance is significantly lessened. This, in turn, would require the use of materials having a comparatively high, relative magnetic conductivity, such as free-machining steel or structural steel. However, such materials can, as discussed, for example, also in U.S. Pat. No. 6,330,832, not always completely satisfy the high requirements placed on industrial-grade, measuring transducers of the type being discussed, as regards corrosion resistance and/or hygiene, so that then measures would have to be utilized further increasing the anyway already high materials- and/or manufacturing-complexity.